Photovoltaic junctions are photon-energy dependent. Typically, photons are absorbed, and electron-hole pairs created, only when an arriving photon is at least of a particular minimum energy, roughly corresponding to an energy bandgap of the photon-absorbing layer of the photovoltaic device.
The energy gap of the photon absorbing layer also relates to the maximum generated voltage output of the junction—the higher the bandgap, the greater the output voltage and the greater energy absorbed from each captured photon.
Photons having energy greater than the minimum energy for electron-hole creation tend to create heat in the absorber along with single electron-hole pairs, at typical optical energies they do not create additional electron-hole pairs, and do not boost voltage output.
Photovoltaic junctions therefore tend to act as optical low-pass filters, being transparent to photons having energy having less than the minimum energy associated with the absorbing layer; in typical single junction devices any energy carried by those photons is wasted.
Stacked-junction photovoltaic devices can take advantage of low-bandgap absorber layers to absorb more low-energy photons, while taking advantage of high-bandgap materials to capture more of the energy available in high-energy photons. By doing so, stacked junction devices can theoretically capture a greater percentage of the energy available in sunlight than single junction devices
While stacked-junction photovoltaic devices are known in the art, existing devices are expensive to make and are typically not monolithically integrated into multiple-stack modular arrays. Stacked-junction devices have been made by attaching an upper photovoltaic cell complete with its own separate metal interconnect and the substrate on which it was fabricated, above a lower cell also complete with its own interconnect and substrate, a bonding agent or adhesive may be used to hold those upper cells to the lower cells. Stacked-junction devices have also been made by depositing additional thin-film semiconductor layers over a lower photovoltaic cell to form an upper photovoltaic cell overlying the lower photovoltaic cell, these devices have no bonding agent or adhesive between upper and lower cells.
Photovoltaic devices, such as some I-III-VI semiconductor-based devices, including some CIGS (Copper-Indium-Gallium-Selenide) cells, have been successfully fabricated on flexible metal or polymer substrates instead of traditional rigid substrates. These cells have ad-vantages in weight over, and may resist damage from some hazards better than, cells on traditional rigid substrates like silicon or germanium wafers or glass.
Microlink Devices, of Niles, Ill. has developed gallium-arsenide (GaAs) based photovoltaic devices on epitaxial lift-off wafers; see LIGHTWEIGHT, LOW COST GaAs SOLAR CELLS ON 4″ EPITAXIAL LIFTOFF (ELO) WAFERS, R. Tatavarti, et al. 33rd IEEE photovoltaic specialist conference, 2009 (Tatavarti). These devices are fabricated by depositing the thin films of a III-V semiconductor-based photovoltaic cell over an Aluminum Gallium Arsenide (AlGaAs) release layer on rigid, reusable, substrates. The AlGaAs layer is then etched away to release the cell from the substrate. The resulting flexible cell is reported as functioning well despite being rolled around a 35 mm mandrel.
Thin, flexible, photovoltaic devices can be adhered to a rigid or flexible substrate for use in standard rooftop applications. These flexible devices, however, can also be adhered to a flexible substrate and encapsulated with flexible materials that are not only lighter in weight than most rigid photovoltaic assemblies, are usable in stationary or rooftop applications, but can also often be folded or rolled for portability. A market for portable photovoltaic devices exists among expedition-level hikers, campers, recreational vehicle users, and the military. These devices may be used by small units for recharging batteries in the field for use in illuminated gunsights, radios, computers, and electronic map devices, navigational equipment, sensors, lights, and other devices.